Heretofore, heating systems for residential use have included heat exchange devices in which a fluid is passed through metal passageways heated to relatively high temperatures by combustion gases rising from a heat producer such as a gas burner.
In the case of a typical forced-air furnace, the exterior surface area of the exhaust channels radiate the heat from inside of the heat exchanger. The heated air is directed by an air handler into the warm-air plenum of a network of ducts in the residence. The burned gas passes through the inside of the heat exchanger to the vent outlets and then to the stack.
Boilers for typical hot water heating systems contain boiler sections kept filled by circulating water. The hot combustion gases from the burner flow past the water filled sections, heat them and escape via a stack. Heating efficiency in this type of system results from heating the water to above boiling, for example, from 200.degree. to 230.degree. fahrenheit. The water does not become steam because it is under pressure in the system.
In either of these systems the furnace or boiler usually consists of elongate hollow metal sections which may be flat or somewhat curved and which have relatively large interior and exterior surface areas. Such heat exchangers generally work well, but require large quantities of heat and thus consume proportionate quantities of fuel. Accordingly, heat efficiency in these systems is achieved at the expense of cost efficiency.
Previously known coil-type heat exchangers have been thought to exhibit disadvantages. Examples of such heat exchangers are set forth in U.S. Pat. Nos. 2,823,652 to C. K. Mader, 3,267,909 to B. E. McClanahan and 3,612,004 to E. Cancilla et al. These patents describe helical and ring-type heat exchange tubes provided with exhaust baffle elements or deflectors which extend between adjacent turns of the coil or which are otherwise positioned to block or interfere with the flow of combustion gases through the coil. Baffles of this type are provided to recycle the combustion gases over the coil or to compensate for coil distortion resulting from expansion and contraction which would otherwise tend to open the exhaust passages. However, the baffles or deflectors diminish the velocity of the exhaust gas molecules flowing by the coil. This means that heat exchange efficiency between the exhaust gas molecules and the surface area of the coil is reduced. Moreover, the coils and baffles will tend to coke-up with time thereby further reducing heat exchange efficiency. Accordingly, the heat producer is selected so as to provide an enormous quantity of heat, sometimes on the order of 2,000,000 BTU's per hour. These arrangements, therefore, including the tube-type heat exchanger described in U.S. Pat. No. 3,734,065 to E. A. Reid, Jr. et al are designed for high output heat producers where a large volume of a heated fluid is required, as for example in swimming pools or for commercial use in the petroleum and chemical industries.